Low noise electron beam plasma amplifier



July 30, 1963 J. M. ANDERSON 3,099,768

LOW NOISE ELECTRON BEAM PLASMA AMPLIFIER Filed March 25, 1959 In vemorJohn M. Anderson,

His A Home y.

United States Patent 3,099,7 68 LOW NOISE ELECTRON BEAM PLASMA AMPLIFIERJohn Melvin Anderson, Scotia, N.Y., assignor to General ElectricCompany, a corporation of New York Filed Mar. 25, 1959, Ser. No. 801,8523 Claims. (Cl. 315-5.39)

This invention relates to an electron discharge apparatus and moreparticularly to an electronic amplifier device utilizing an electronbeam-gaseous discharge plasma interaction, for amplifyingelectromagnetic wave signals.

In the operation of various electronic apparatus, it is usuallynecessary to amplify various signals to more tractable magnitudes forallowing various functions to be performed on the signal or forsatisfactorily operating various of the circuit devices involved in theapparatus. It is important in increasing the amplitude of a desirablesignal to achieve such amplification without introducing noise on thesignal as well as to prevent amplification of noise already superimposedon the signal. A wide variety of electronic amplifiers of differenttypes are known for performing such functions and in conventionalamplifier circuits, various techniques have been devised for achievinghigh signal amplification without concomitant noise amplification. Inamplifiers exploiting the interaction of an electron beam with theplasma of a gaseous discharge for achieving amplification, however,larger signal-to-noise ratios are desirable.

It is accordingly a principal object of my invention to achieve highamplification and a high signal-to-noise ratio in electromagnetic wavesignal amplification utilizing the interaction between an electron beamand a gaseous discharge plasma.

In accordance with my invention and pursuant to the aforementionedobject, a density modulated beam of electrons is projected through agaseous discharge plasma with which it interacts to amplify themodulations on the beam and in accordance with a particular feature ofmy invention, for minimizing the introduction of noise in the outputsignal of the amplifier the interacting plasma utilized is that in thenegative glow portion of the discharge. Thus, the interacting electronsof the plasma in this region which are of relatively low temperature andlow energy, contribute only negligibly to production of noise in theamplification process.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which I regard as myinvention, the invention may be better understood from the followingdescription taken in connection with the accompanying drawing in which:

FIG. 1 is an elevation view insection illustrating the construction of apractical form of device according to my invention, and

FIG. 2 is an end view, partly in section, of the device shown in FIG. 1.

Referring now more particularly to the drawing for a detaileddescription of the invention, in the figures, represents generally anamplifier according to my invention including an electron gun designatedgenerally at .12, an input cavity 14, an interact-ion chamber 16 filledwith an ionizable gas at low pressure, an output cavity 18, and anelectron collector 20. These components are sealed together at adjoiningsurfaces to form a sealed enclosure which contains the ionizable gas atlow pressure as hereinbelow more fully described.

The electron gun 12 comprises an electron emissive, coated cathode 22, aheater 24 for heating the cathode to a temperature of oopius thermionicemission and a plurality of three beam forming or acceleratingelectrodes 3,099,768 Patented July 30, 1963 "ice 26, 2'8 and 30 of diskshape and being centrally apertured to accommodate the passage ofelectrons. The cathode 22 is secured to a conductive rod 32 and heater24 is connected at its ends to conductive rods 34 and 36. For supportingthe cathode 22 and heater 24, the rods 32, 34 and 3'6 are embedded in anend wall 38 of an envelope shown generally at 40. For supportingelectrodes 26, 28 and 30, a clamping ring 42 is tightly fitted about aninwardly protruding portion 44 of wall 38 and has a plurality ofprojecting arms such as 46 and 48 extending along a portion of theenvelope. Rigidly secured to the ends of the arms 46 and 48 arecorresponding extensions such as shown at 50 and 52, of glass or othersuitable insulating material to which the electrodes 26, 28 and 30 aresecured at spaced locations. More than two such electrode supports maybe provided as is desired or necessary. The proper potentials may beapplied to the accelerating electrodes through conductive wires 54, 56and 58, leading to respective electrodes 26, 2'8 and 30 Such potentialsand the potential of cathode 22 may be derived from a source of directpotential shown as a battery 60 connected across a resistor element 62and to which the cathode and respective electrodes are connected atdifferent points. The positive most portion of the element 62 isgrounded, the cathode is connected to the negative most portion thereofand the accelerating electrodes are connected at points intermediate tothe positive most and negative most portions. Suitable heater currentmay be provided for heater 24 in any conventional manner.

The electrons produced by gun 12 are attracted to collector 20 which isgrounded and therefore at a high positive potential with respect tocathode 22 and the electrons in their beam path, first traverse inputcavity 14 which is a hollow, annular member with inwardly directed, diskshaped member 64 and 66 extending from a circular opening in the cavityand the disks are apertured at a central region thereof but haverespective screen grids 68 and 70 covering such apertures. An inputsignal may be introduced into the cavity to excite the same by anysuitable wave guide and coupling means such as the coaxial wave guide 72having the inner conductor 73 thereof terminating in coupling loop 74.The outer conductor 75 of wave guide 72 is directly connected to theconductive Wall of cavity 14' but the inner conductor 73 is insulatedtherefrom along portions remote from its end by suitable insulatingmaterial 76.

In response to excitation of cavity 14 by the introduction of a signalof the proper frequency, the grids 68' and 70 vary in potential withrespect to each other in a sinusoidal manner, whereby electronstraversing the grids from gun 12, are selectively accelerated andretarded to produce bunching as they leave the gap between the grids anddrift into the chamber 16.

The chamber 16 comprises a round enclosure closed by end walls 78 and 80and envelope 40 is secured to a circular flange 82 eccentricallypositioned in wall 78. An opening 83' concentric with flange 82 isformed in wall 78 and protruding from the Wall 78 toward gun 12 is atubular sleeve 84 serving as a surface for diffusion of chargedparticles which prevents positive ions present in chamber 16 fromentering the region between grids 68 and 70 to confine positive ionsWithin the chamber 16. It is to be understood that other and dilferention traps such as auxiliary grids may be utilized equally well. Forproducing a gaseous discharge in chamber 16, as shown in FIG. 2, as wellas in FIG. 1, an arcuate, cold cathode 85 is disposed along a portion ofthe interior of chamber 16 remote from the eccentric opening in thechamber and a direct potential difference is applied across the anodechamber and cathode derived from a source repre- 3 sented by a battery86. The cathode 85 is supported by a conductive rod 88 which is securedto the cathode at one end and sealingly bonded to an insulator 90' whichin turn is sealingly bonded to a tubular supporting bracket 91 extendingfrom the chamber 16.

In alignment with the envelope 40 and on the side of chamber 16 remotetherefrom is disposed an envelope 92 which is sealed at one end to aflange 94 extending about an opening 96 in wall 80 and in coaxialalignment with opening 83. The collector 20 is supported on a conductiverod 98 which extends through an end wall 100 of the envelope 94 and issealed thereto and for the present purposes, the rod 98 is connected toground.

Output cavity 18 is also mounted on envelope 92 and comprises a hollow,annular member with a pair of inwardly directed disks 102 and 104extending from a circular opening in the cavity and the disks arecentrally apertured at 106 and 108 with respective screen grids 110 and112 covering such apertures. Bunches of electrons in the beam betweencathode 22 and collector 20 traverse the gap between grids 110 and 112and excite the cavity 18. For removing energy from the cavity 18, acoaxial wave guide 113 is provided with its outer conductor 114connected to the conductive wall of cavity 18 and its inner conductor116 terminating in a loop 118 within the cavity and connected at its endto an interior wall of the cavity. Along portions of the inner conductorremoved from its end, it is maintained in nonoonductive relationshipwith the cavity by an insulator 120 sealingly bonded to both the innerconductor and: an opening in the cavity.

In constructing the amplifier 10, it is evacuated and filled with anionizable gas such as helium at a low pres sure of the order of .1 mm.of mercury. Other gases may be utilized and such pressure may be variedover a range, it being necessary, however, to maintain the pressuresufii- Iciently low that the mean 'free path within the gas is greaterthan the distance across the anode chamber between grids 68 and 70 andgrids 110 and 112, since it is this region that contains any appreciablenumber of positive ions and it is necessary for proper operation of theamplifier for a predominance of the electrons from gun 1-2 to passthrough the anode chamber 16.

In the operation of the amplifier '10, appropriate potentials areapplied to cathode 84, anode chamber 16, cathode 22, electrodes 26, 28and 30, collector 20 and heater 24 to produce both ionization of the gaswithin the anode as shown at 122 and an electron beam between cathode 22and collector 20. In accordance with a feature of my invention, theionizing potential, the anode dimensions and gas pressure within theanode produce a gaseuos discharge within the anode in a condition ofnegative glow in the electron beam path in anode chamber 16. In such adischarge the ultimate electrons, which are those that are produced .byelectron-atom collisions, may possess low values of mean energy of theorder of .2 electron volts or less. This corresponds to a temperature ofapproximately 770 K.

An input signal is introduced into cavity 14 through coaxial wave guide72 and the excitation of the cavity produces varying potential at grids68 and 70 to produce bunching of the electrons in the beam from gun 12.As the bunches of electrons drift into the anode chamber 16, aninteraction the exact nature of which is presently not known in alldetails, occurs between the negative glow portion of the ionized gas andbeam bunches to greatly accentuate the bunching or in other words toincrease the bunch density and in effect to amplify the effect of the.input signal on the beam. Due to the relatively low energy of theelectrons in the negative glow region, introduction of noise on the beamis maintained at a minimum.

The accentuated bunches of the beam continue in their path through thegrids 110 and 112 associated with cavity 18 and excite the cavity topermit removal of energy therefrom through coaxial wave guide 113 andsuch bunches are finally collected at collector 20.

An amplifier such as herein described is most effective at inputfrequencies of the order of the plasma frequency of electrons in theionized plasma. The plasma frequency is the frequency of oscillationthat would result if a group of electrons were separated from thepositive ions in a predetermined volume of space and then allowed to return. In accordance with my invention, densities of low temperature andlow energy electrons in a negative glow region of a discharge areobtainable to achieve effective amplification of signals at frequenciesfrom 1,000 megacycles per second to 100,000 megacy cles per second whichcorrespond to electron densities of 10 to 10 electrons per cubiccentimeter, respectively.

While the present invention has been described by reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the invention, I, therefore, aim in the appended claimsto cover all such equivalent variations as come within the true spiritand scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An apparatus comprising a chamber filled with an ionizable gas at lowpressure, means ionizing a region of said gas extending entirely throughsaid chamber into a condition of negative glow, means establishing adensity modulated electron beam through said region, said beaminteracting with the said ionized gas in said region of negative glow toaccentuate said modulation and amplify the effects of said signal onsaid beam and means tor extracting said signal from said beam subsequentto said interaction.

2. An apparatus compirsing a chamber filled with an ionizable gas at lowpressure, means ionizing a region of said gas extending entirely throughsaid chamber into a condition of negative glow, means including anelectron gun and electron collector on difierent sides of said chamberproducing an electron beam through said region, an input resonant cavityhaving bunching grids between said gun and said chamber and interposedin the path of said beam, means for exciting said cavity by an inputsignal to density modulate the electrons in said beam, an outputresonant cavity having bunching grids interposed in the path of saidbeam on the side of said chamber remote from said first cavity, andmeans coupling an energy propagating wave guide to each of said resonantcavities.

3. An apparatus comprising a chamber filled with an ionizable gas andmeans for projecting an electron beam along a predetermined path throughsaid chamber, the mean free path in said gas being greater than thelength of path therethrough followed by said beam, means for densitymodulating the electrons prior to their entrance into said chamber toproduce bunching thereof, means for ionizing the region of said gasalong .a predetermined path to a condition of negative glow discharge,the electrons in said bunches interacting with said ionized gas in saidregion to accentuate the bunching of said electrons and means excited bythe beam of accentuatedly bunched electrons for removing an amplifiedsignal from said beam.

References Cited in the file of this patent UNITED STATES PATENTS2,806,974 Haefl? Sept. 17, 1957 2,824,997 Haelf Feb. 25, 1958 2,848,649Bryant Aug. 19, 1958 OTHER REFERENCES Gaseous Conductors, by J. D.Cobine, 1st edition, 1941, pages 206, 212, 213, and 214, McGraw-HillBook, Co, New York.

1. AN APPARATUS COMPRISING A CHAMBER FILLED WITH AN IONIZABLE GAS AT LOWPRESSURE, MEANS IONIZING A REGION OF SAID GAS EXTENDING ENTIRELY THROUGHSAID CHAMBER INTO A CONDITION OF NEGATIVE GLOW, MEANS ESTABLISHING ADENSITY MODULATED ELECTRON BEAM THROUGH SAID REGION, SAID BEAMINTERACTING WITH THE SAID IONIZED GAS IN SAID REGION OF NEGATIVE GLOW TOACCENTUATE SAID MODULATION AND AMPLIFY THE EFFECTS OF SAID SIGNAL ONSAID BEAM AND MEANS FOR EXTRACTING SAID SIGNAL FROM SAID BEAM SUBSEQUENTTO SAID INTERACTION.